Microwave energy applicator

ABSTRACT

An apparatus is disclosed for applying microwave electromagnetic energy within a resonant conductive enclosure by means of an array of slots in a waveguide launching section establishing a high field standing wave pattern. This pattern provides high fringing fields across a path in close proximity to the slots. Means for individually or continuously feeding articles to be heated along this path within the conductive enclosure as well as safety interlock electrical circuits are provided in the apparatus. Articles requiring extremely fast heating times such as continuous filaments of synthetic goods, thin film materials or paper products coated with a heat sensitive material, are examples of articles adapted to be heated in the disclosed applicator apparatus.

United States Patent [191 Edgar et al.

1 MICROWAVE ENERGY APPLICATOR [75] Inventors: Richard H. Edgar, Chelmsford;

Robert A. Peterson, Canton, both of Mass.

[73] Assignee: Raytheon Company, Lexington,

Mass.

[22] Filed: Sept. 27, 1972 [2]] Appl. No.: 292,567

[ 51 Nov. 5, 1974 Primary Examiner-J. V. Truhe Assistant Examiner-Hugh D. Jaeger Attorney, Agent, or Firm-Edgar O. Rost; Joseph D. Pannone; Harold A. Murphy [5 7] ABSTRACT An apparatus is disclosed for applying microwave electromagnetic energy within a resonant conductive enclosure by means of an array of slots in a waveguide launching section establishing a high field standing wave pattern. This pattern provides high fringing fields across a path in close proximity to the slots. Means for individually or continuously feeding articles to be heated along this path within the conductive enclosure as well as safety interlock electrical circuits are provided in the apparatus. Articles requiring extremely fast heating times such as continuous filaments of synthetic goods, thin film materials or paper products coated with a heat sensitive material, are examples of articles adapted to be heated in the disclosed applicator apparatus.

3 Claims, 5 Drawing Figures I MICROWAVE ENERGY APPLICATOR BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to microwave heating systems and, more particularly, a high field standing electromagnetic wave applicator for developing fringing energy fields in a concentrated area.

2. Description of the Prior Art Microwave heating of a large number of domestic and industrial products has become widespread due to the rapid heating times provided by the electromagnetic energy generated from such sources as the magnetron which is well known in the radar art. The energy is radiated within an enclosure which is resonant at a predetermined frequency and is ideally suited for inherently poor thermal conductors such as paper, textiles, leather and the like. The energy sources typically operate in the microwave region of the electromagnetic energy spectrum and operating frequencies of 915 and 2450 MHz have been allotted by the cognizant Federal regulatory agency. For the purposes of the present specification, the term microwave is defined as electromagnetic energy radiation in that portion of the spectrum having wavelengths in the order of one meter to one millimeter and frequencies in excess of 300 MHz. The intense electric and magnetic fields result in the perturbation of the molecules of the article being heated by the changing electric and magnetic fields. The constant and rapid movement of the molecules results in the article being rapidly heated by the absorption of the microwave energy commencing from the outside layers.

Certain thin articles, such as films or filaments of synthetic materials or paper products have not been subjected to microwave heating due to difficulties in the application of such energy over very short periods to avoid damage to the product by overheating. Heating with microwave electromagnetic energy is useful in heating or drying coatings deposited on such thin articles or removing residual moisture after processing. In merchandising thin cardboard labels having a surface coated with a heat sensitive adhesive are used in place of the stapled or pinned type for price and inventory data. Such heat sensitive labels require short heating periods to activate the adhesive material and conventional radiant heating sources may damage the article by overexposure.

To date, to the best knowledge of the inventors, no apparatus or system is available which permits the utilization of microwave electromagnetic energy with its inherent advantages of rapid heating times for such adhesive coated articles or synthetic fibrous materials.

SUMMARY OF THE INVENTION In accordance with the present invention, an apparatus for the application of microwave electromagnetic energy in a predetermined standing wave pattern is disclosed. A slot array in the wall of a rectangular waveguide launching section having short-circuited end walls provides a standing wave pattern with high electromagnetic fringing fields along a path closely adjacent to the coupling slots. A shielded resonant conductive enclosure surrounds the slot array and means for feeding the article to be heated are provided for exposure within the path of the high fringing fields for relatively short times of, illustratively, one second or less. The microwave energy applicator includes an energy source, such as a magnetron, DC power supply for converting domestic house current to illustratively 4,000 to 6,000 volts, as well as associated interlocks, switches and relays. The energy source is in a stand-by position after actuation of the'apparatus and the insertion of the article to be heated in one embodiment results in the closing of a contact to energize the shielded enclosure and heat a surface of the article. In the case of heat sensitive merchandising labels only the selected adhesive coated portion is exposed to the microwave energy radiating through a selected slot.

The microwave heating applicator is readily adapted for continuous feeding of the articles being heated and in merchandising applications it is possible to provide for a cartridge type loading operation wherein a plurality of the heat sensitive labels are almost instantaneously heated upon the application of the energy across the span of the energy coupling slot. The articles being heated are positioned in close proximity to the slot having the appropriate dimensions to provide for the concentration of the intense electromagnetic fields in the path of the article being heated.

BRIEF DESCRIPTION OF THE DRAWINGS Details of illustrative embodiments of the invention will be readily understood after consideration of the following detailed description and reference to the accompanying drawings, wherein:

FIG. 1 is a cross-sectional and diagrammatic view of the embodiment of the invention;

FIG. 2 is a plan view of the embodiment of the invention shown in FIG. 1 taken along a line 2-2;

FIG. 3 is a plan view of an illustrative heat sensitive article for use with the illustrative embodiment of the invention;

FIG. 4 is a partial isometric view of a conveyorized feed system for incorporation in the embodiment of the invention; and

FIG. 5 is a diagrammatic representation of the standing wave fringing electromagnetic fields with the microwave applicator of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1 and 2, a microwave heating applicator 10 embodying the invention is illustrated. A section of rectangular hollow pipe waveguide 12 provides the means for launching of the microwave energy generated by conventional magnetron indicated generally by the box l4 operating at the assigned frequency of 2,450 MHz and its associated power supply 16 including a high voltage transformer and DC rectifying circuit. The generated energy is coupled into launching waveguide 12 through an aperture in a top wall by probe antenna 18 housed within dielectric dome member 20. The electrical circuits for the timers, controls, as well as safety interlocks for actuation of the overall system have been omitted for the sake of clarity since they are well known in the art.

Waveguide launching section 12 is terminated at one end by a wall member 22 spaced from the antenna 18 a predetermined distance to provide for optimum launching of the energy. The opposite end of the waveguide section is terminated by a short circuiting end wall member 24 which provides a means for reflecting of the launched energy to establish a high standing wave pattern. Adjacent to the end wall member 24 is an array of slots 26, 28 and 30 of varying dimensions in the top wall of the waveguide section. The microwave energy, therefore, within the waveguide section is provided with an access outside of the section by the array of slots with the most intense electromagnetic fields being established across the widest and longest slot 30 which is in close proximity to the end wall member 24. It is noted that the slots 26 and 28 are varied in such a manner that the maximum electromagnetic fields are established across the slot 30.

Referring now to FIG. a diagrammatic representation of the high electromagnetic fields in the vicinity of the slots is illustrated. Slots 26, 28 and 30 have been similarly identified as the structures in FIGS. 1 and 2. Field lines 32 span the slot 26 and field lines 34 extend across slot 28. Slot 30 which is the widest and longest provides for the energy field lines 36 in the vicinity of the opposing walls of the top wall of the waveguide adjacent thereto. A region of high electric field concentration, therefore, is defined along the path closely adjacent to the largest slot 30.

Referring again to FIGS. 1 and 2, a resonant shielded conductive enclosure 40 surrounds the outer walls of the waveguide 12 in the area of the array of slots. A hinge 42 is secured to the top wall of the waveguide section 12 and a handle 44 facilitates the raising and lowering of the enclosure 40.

An article to be heated of the type ideally suited for use with the disclosed microwave heating applicator is a merchandising multi-part perforated label 50 and one side is shown in FIG. 3. This label is of the heat activated adhesive coated type and is readily adapted for many types of merchandising applications. The multipart label is formed of a thin cardboard paper composition. The first portion 52 is joined to the central portion 54 by a perforation 56. A final portion 58 is provided with a crease line 60. Sheets of a plurality of labels may be printed and are joined together by tabs62 on opposing sides. ,T he labels may be printed on one side. The underside of portion 52 is blank the outer side may be used for printing any advertising or any other informative matter. The side shown of portion 54 is coated with a color which may be useful in indicating certain information for inventory and accounting and the reverse side may bear suitable printed code numbers. The label is provided with apertures 64 and 66 for ease in storage at check out counters by spindle or other means. The label portion 58 may carry the style, code and price in formation on the reverse printed side while the underside shown as well as a part of the central portion 54 is coated with a heat activated adhesive material 68. After the label has been heated portion 58 is folded along crease line 60 to form a U or V-shape and is placed on a garment sleeve or other appropriate position. The adhesive material 68 is thereby joined to both sides of the garment material to which the label is affixed.

The article shown in FIG. 3 is positioned in the region of the high fringing field path adjacent to one side of slot 30. As shown in FIG. 1, the label 50 is supported by slide members 70 which may be of a lossy dielectric material, such as that commercially available under the tradename Teflon. Label 50 is inserted through access means in a wall of shielded enclosure 40 such as a slot and is inserted in the direction parallel to end wall members 22 and 24 with the adhesive coating 68 exposed to the microwave energy escaping through the slot 30. In FIG. 2 the label 50 has been shown over the slot and with the insertion of labels individually a trip switch 72, electrically coupled to the power supply and magnetron circuit by lead line 74, rapidly energizes the circuit when contacted by the end portion 58 of the label. The microwave energy almost instantaneously radiates the adhesive coated portion of the label upon the closing of contacts within the switch 72. In applications where a plurality of tags can be utilized at specific times a continuously fed arrangement will be more desirable. In FIG. 4 a section of rectangular waveguide 76 and superimposed enclosure 40 is illustrated with the sidewalls of the enclosure 40 partially broken away to reveal the internal feed mechanism. A roll 78 of a plurality of labels 80 is positioned outside of the waveguide and one end is fed through the walls of the enclosure through appropriate slots 82. A push button control 84 with suitable controls indicated generally by the box 86 will provide for the actuation of the circuit which will activate the microwave energy generator. Alternatively, a foot control could be employed. The plurality of labels 80 is maintained in the desired slot position for heating by spaced groove rails 88 of a lossy material. After the labels have been heated the operator simply may pull the group from the microwave energy applicator which automatically reloads the slot section for the next cycle. Alternatively, a mechanized feed mechanism with appropriate moving and positioning means may readily be incorporated in the disclosed apparatus.

A simple, relatively inexpensive, as well as readily portable apparatus is disclosed which facilitates very rapid heating of articles not capable of being exposed to radiant heat for any length of time. It will also be obvious to those skilled in the art to modify the apparatus to heat any article by exposure in the concentrated area of high fringing electromagnetic energy fields. .A broad interpretation of the foregoing description of the invention as defined in the appended claims is, therefore, intended.

We claim:

l. A microwave energy heating apparatus comprising:

a source of electromagnetic microwave energy;

means for propagating said energy including a waveguide having a longitudinal axis closed at both ends to establish standing waves with the axis of propagation extending along said longitudinal axis;

said waveguide having an array of spaced slots in one wall with each of said slots having a gradually varying length substantially greater than the width with the major dimension extending transverse to the axis of the propagation of energy to provide high fringing electromagnetic fields along a path in close proximity to said slots;

a conductive enclosure positioned on said slotted waclil to confine energy escaping through said slots; an

means for positioning an article within said enclosure to be exposed to energy from only one of said slots.

2. The apparatus according to claim 1 wherein said array of slots vary in length and width to provide the highest intensity energy through the slot furthest away from said energy source.

3. The apparatus'according to claim 2 wherein said article to be heated is positioned adjacent to only said slot having the high st intensity energy. 

1. A microwave energy heating apparatus comprising: a source of electromagnetic microwave energy; means for propagating said energy including a waveguide having a longitudinal axis closed at both ends to establish standing waves with the axis of propagation extending along said longitudinal axis; said waveguide having an array of spaced slots in one wall with each of said slots having a gradually varying length substantially greater than the width with the major dimension extending transverse to the axis of the propagation of energy to provide high fringing electromagnetic fields along a path in close proximity to said slots; a conductive enclosure positioned on said slotted wall to confine energy escaping through said slots; and means for positioning an article within said enclosure to be exposed to energy from only one of said slots.
 2. The apparatus according to claim 1 wherein said array of slots vary in length and width to provide the highest intensity energy through the slot furthest away from said energy source.
 3. The apparatus according to claim 2 wherein said article to be heated is positioned adjacent to only said slot having the highest intensity energy. 